Fertility refers to the actual number of live births in a given period relative to the size of the population (as distinct from the physical ability to reproduce). The total fertility rate (TFR) is a measure of current fertility. Australia's TFR dropped below replacement level (2.1 babies per woman) in 1976. It has remained below replacement level and declined further since then. This means that under current age-specific fertility rates the average number of babies born to a woman throughout her reproductive life would not be sufficient to replace herself and her partner.

During the 1970s, Australia's TFR declined rapidly from 2.9 babies per woman in 1971 to 1.9 in 1981. In more recent years this decline has slowed, with the TFR reaching 1.8 by 1996. Since then, the TFR has been relatively stable, varying between 1.7 and 1.8 (graph 5.33). In 2003 there were almost five million women of child-bearing age in Australia. About 5% of these women gave birth in 2003, a decline of half a percentage point since 1993.

This article discusses three of the main drivers of low fertility in Australia - the changing age of mothers, declining family size and childlessness.

DELAYING CHILDBIRTH

The current low level of fertility in Australia is related to the fact that partnering, and consequently childbirth, is occurring at later ages than in the past. The age at which women begin bearing children is a significant determinant of lifetime family size. With fewer reproductive years available, women who start having children later in life tend to have fewer children than those who start having children at younger ages (Barnes, 2001). Delaying child-bearing also increases the risk of lifetime childlessness (Weston, 2004).

The trend towards delaying child-bearing in Australia is evident in the median age of parents. For both mothers and fathers, median age has risen consistently over the past two decades. In 1983, the median age of all mothers who gave birth in that year was 26.9 years, while the median age of fathers was 29.7 years. By 1993, these had increased to 28.9 years for mothers and 31.4 years for fathers. Ten years later, in 2003, the median ages were 30.5 years and 32.6 years for mothers and fathers respectively.

Over this period, declines in fertility rates in younger age groups (15-29 years) have not been fully offset by increases in fertility in older age groups (30-49 years) (DeVaus, 2002; Kippen, 2003; McDonald, 2000). This has resulted in a gradual decline in total fertility levels, and an increased median age of mothers (graph 5.34). In 2000, this shift towards older ages of mothers resulted in the age group with the highest fertility rates shifting from 25-29 year olds to 30-34 year olds.

Fertility of the 25-29 year age group decreased by 21% between 1993 and 2003. In 1993, women in this age group had 129.8 births per 1,000 women, declining to 102.9 in 2003. The 25-29 year age group accounted for 35% of the TFR in 1993, but contributed only 29% in 2003.

Accompanying this decline has been an increase in the fertility of women aged 30-34 years and 35-39 years. Since 2000, women in the 30-34 year age group have experienced the highest fertility of all age groups, overtaking women aged 25-29 years. In 2003, there were 112.5 births per 1,000 women aged 30-34 years, a 7% increase from 105.4 in 1993. The contribution by women in this age group to the TFR increased from 28% in 1993 to nearly a third in 2003.

Between 1993 and 2003, the fertility rate for women aged 35-39 years increased by 40%, reaching a high of 54.3 births per 1,000 women. The contribution to the TFR of women in this age group increased from 10% in 1993 to 15% in 2003, nearly equal to the contribution by women aged 20-24 years in that year (16%).

FAMILY SIZE

The declining number of women who have given birth to three or more children in their lifetime is another factor contributing to Australia's low fertility level (Barnes, 2001). In 2000, it was estimated that 25% of that years TFR was contributed by women having a third or higher order birth (ABS, 2000). Without these women, the TFR in that year would have been 1.3 rather than 1.7 births per woman.

As most children are born to women aged under 40 years, the number of children already born to women aged in their 40s is a good indication of the number of children they will ever have. The proportion of women aged 40-49 years with three or more children declined from 54% in 1976 to 46% in 1986 and 37% in 1996. Over this period, the proportion of women having only two children increased from 24% in 1976 to 30% in 1986 and 37% in 1996. Similarly, the proportion with one child increased from 8% to 10% over the period.

CHILDLESSNESS

The number of women who, whether by choice or circumstance, had not given birth to a child also increased significantly between 1976 and 1996. The 1976 Census of Population and Housing recorded 4% of women aged 40-49 years had not given birth to a child. By 1986 the proportion had increased to 9% and in 1996, 11% of women in this age group had not given birth. A recent Australian survey on fertility decisions found that only 8% of surveyed women without children definitely did not want children (Weston et al, 2004).

The proportion of women aged under 30 years who have not given birth has also increased as women delay child-bearing. For instance, of women aged 25-29 years in 1976, 15% had not given birth compared with 40% of women the same age in 1986 and 54% in 1996.

The continued delaying of births may result in lifetime childlessness for some women, despite their fertility intentions. Childlessness among women who have not yet completed their reproductive years can only be estimated. In 2000 it was estimated that 24% of women who had not completed their fertility would remain childless for life if 2000 fertility rates remained constant into the future (ABS, 2000).

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